The most common type of waveguide propagates signals in only one specific electromagnetic field pattern or mode, out of an infinite number of possible modes. Single-mode operation occurs because the waveguide is designed so that signals are in a frequency band which is sufficiently low that only the mode with the lowest "cutoff frequency" can exist and no other mode can propagate. If other modes were allowed to propagate, signal energy could couple into and out of various modes substantially distorting the signal. Such "conventional waveguide" is compact and easy to design, model and use. Unfortunately, maintaining only the lowest-cutoff mode in a given frequency band requires restriction of the waveguide cross section dimensions, and this, in turn, restricts power carrying capacity and limits the lowest achievable signal attenuation. As a result, design of some systems requiring microwave or millimeter wave signal transmission with high power or very low loss may be difficult or impractical.
An alternative type of waveguide is generally called "overmoded" in which a higher order mode is used, i.e. a mode which does not have the lowest cutoff frequency. Because other (unwanted) modes are also capable of existing as well as the desired transmission mode, this type of waveguide must feature internal structures which suppress the unwanted modes. Because internal structure, rather than restriction of cross section dimensions, is the basis for suppressing all but the desired mode, overmoded waveguide cross section can, in principle, be made arbitrarily large for a corresponding increase in power capacity and decrease in signal attenuation. Unfortunately, this type of waveguide, with unwanted mode suppression, is difficult to model and design, and its cross-sectional dimensions may not be amenable to compactness without significant design optimization.
Historically, the more successful type of overmoded waveguide supports the circular TE.sub.01 mode and uses either a dielectric lining or dielectric sheathed helix of insulated wire inside the circular cross section waveguide for suppression and decoupling of unwanted modes, e.g. see A. E. Karbowiak "Trunk Waveguide Communication", Chapmen and Hall Ltd. 1965. Both versions of overmoded TE .sub.01 waveguide were originally developed and tested for millimeter band (60-100 GHz) trunk line telecommunications between cities. Application of overmoded waveguide technology for high power and/or low loss transmission in microwave or millimeter wave radio communications an radar has also been suggested and developed to a limited degree, e.g. see R. M. Collins "Practical Aspects of High Power Circular Waveguide Systems" NEREM Record, Session 24, pp 182-183,(1962).
Because transmitters, receivers, and antennas normally use standard rectangular waveguide, various transition structures have been developed for interconnecting circular overmoded waveguide and the rectangular waveguide. For example, the well-known Marie transducer has one circular waveguide port and one rectangular waveguide port, and it is referred to as a transducer because it changes the mode of propagation from circular TE .sub.01 to rectangular TE.sub.10. Another transition, called a multiport transducer, has one circular waveguide port and multiple rectangular waveguide ports. The primary advantage of a multiport transducer is that it can handle higher power levels than the Marie transducer. A detailed description of such a multiport device is contained in U.S. Pat. No. 4,628,287.
Waveguide bends are also required for practical overmoded waveguide systems, and several bend structures have previously been proposed. More specifically, a bend could be constructed from helical waveguide; however, such a device must be relatively large to prevent mode conversion. In practice, a 90.degree. helical waveguide bend for S-band application has been fabricated that is approximately eight feet long. For certain applications, such as shipboard use, this is still much larger than desired, and further size reductions are probably not possible without incurring unacceptable losses. Moreover, helical waveguide bends tend to be very expensive to fabricate.
Another type of bend proposed for overmoded waveguide application involves the use of a pair of multiport transducers interconnected by a plurality of rectangular waveguides that are bent at the desired angle, see U.S. Pat. No. 4,679,008. This bend has higher power handling capability than the Marie-based bend, but it requires precise rectangular bend configurations to maintain proper phasing if more than two rectangular waveguides are used.